1. Field of the Invention
The present invention relates to noise elimination from acoustic signals, and more particularly, to means for cancelling variable-frequency self-generated noise from acoustic signal tracking systems.
Acoustic target seekers are in use for locating and tracking the sound of certain attacking weapons, for example. If the seeker system is in a mobile vehicle, which could be an aircraft, submarine, tank, or the like, self-generated vehicle noise interferes with target information signals. A common source of such noise is the propulsion engine of the seeker vehicle, which may be running at various speeds or idling under certain conditions. The present invention will also serve in other kinds of acoustic signal-handling systems.
2. Description of the Prior Art
A notch filter has been suggested in the past, but its frequency response would have to be so broad, due to variable engine speeds, that target frequencies would be lost.
U.S. Pat. No. 3,071,752 discloses an interference reduction apparatus wherein interference noise is pre-recorded and later, in operation, played back to subtract from the total signal plus noise. This method is unsatisfactory for real-time operation.
U.S. Pat. No. 3,599,142 is for the same general purpose as the present invention, showing a control apparatus having phase-locked loops following the frequency of each interfering noise signal. The output of each phase-locked loop is treated by amplitude and phase determining means and later subtracted from a total input signal containing both information and noise. This patent system needs such amplitude and phase determining means for each frequency before the noise subtraction will work, thus adding complications.
While it is possible that more pertinent prior art exists, applicants' search is believed to have been conducted with a conscientious effort to locate and evaluate the most relevant art available at the time, but this statement is not to be construed as a representation that no more pertinent art exists.